Electric brakes for a vehicle, in particular an aircraft, generally comprise at least one electric actuator having a pusher actuated by an electric motor for selectively applying a force against friction elements of the brake.
In order to block the vehicle, e.g. when it is stationary on a parking space, the pusher of the actuator is put into a position in which it exerts a controlled force on the friction element, and then the pusher is blocked in this position so that it continues to exert a parking force on the friction elements, thereby allowing the power supply to the motor to be switched off and thus reducing the electrical power consumption of the brake.
Nevertheless, given the dimensional variations of the components of a brake as a function of temperature, the parking force varies over time, and it is therefore necessary to provide for adjusting the position of the pusher.
In particular from documents U.S. Pat. No. 6,959,794 and FR 2 880 603, it is known to adjust the applied parking force at given instants, either by using a force sensor mounted in one of the components of the braking system and ensuring that the force is maintained at a constant value by means of a closed loop acting on the position of the pusher as a function of the value of the force, or else by implementing a so-called “return-to-zero” method including the steps of unblocking the pusher, retracting the pusher to zero force, and then advancing it once more until it exerts the required force as measured from one of the operating parameters of the actuator, e.g. the power supply current drawn by the electric motor of the actuator.
Those two means consume electricity, and that can pose a problem insofar as when parked, the braking system is powered solely from the vehicle battery.
Above-mentioned document U.S. Pat. No. 6,959,794 also discloses adjusting the position of the pusher in compliance with a so-called “direct adjustment” table that causes the position of the pusher to be varied as a function solely of the length of time that has elapsed since the parking brake was applied. In order to ensure that the parking brake remains effective regardless of circumstances, it is then necessary for the adjustment table to correspond to compensating for the effects of emergency braking, i.e. a situation in which the components are subjected to a large amount of expansion shortly after applying the parking brake. This expansion gives rise in particular to very considerable lengthening of the torsion tube that carries the disks. With the pusher locked in a position that is stationary relative to the torsion tube, a lengthening of the torsion tube gives rise to a reduction in the braking force. In order to compensate for this reduction, it is therefore necessary to provide a direct adjustment table that causes the pusher to be moved considerably in the tightening direction.
Since the adjustment is the same regardless of the circumstances preceding application of the parking brake, there is a risk in certain circumstances of performing adjustments too frequently or at too high a level, such that the adjustments are of no use, particularly after braking at very low energy levels in which the thermal expansion of the torsion tube is small. These useless adjustments contribute to prematurely aging the actuators.